os.walk() yields symlinks to directories in dirnames and do not follow
them. Our backup cycle expects all files that need to go to blob to be
in filenames and that dirnames are only recursed-into by walk().

Thus, until now, symlink to a directory was simply ignored and not
backup'ed. In particular *.git/hooks are usually symlinks to common
place.

The fix is to adjust our xwalk() to always represent blob-ish things in
filenames, and leave dirnames only for real directories.

/cc @kazuhiko